Forum

[Natffs]

Hi everyone, I have noticed when I use the standard plug B7HS it only goes half way down the spark plug thread in the head. Is this correct or has a 500 head been fitted and should I be using a long reach plug ?

[RoSy]

My 500 is a long reach. I'm not sure if any of the bullets are short reach.

[Natffs]

Hi the 350 is a short reach according to Hitchcocks!

[stinkwheel]

The standard 350 is short reach. As he says, B7HS. Standard on the 500 is long reach B8ES.



Has it been twin-plugged? The decompressor hole (right side of the cylinder head as you sit on the bike) would take a long reach plug. The sparkplug hole (left hand side) would take a short reach plug.

[Natffs]

No not twin plugged and the decompressor is working great. Looks like 500 head so should I use a long reach with a 7 heat rating?

[stinkwheel]

Is it definately a 350 then? I would have thought a 500 head would be a poor fit on a 350 barrel spigot. Has someone bunged a full 500 top end on your motor? It's not unusual to get a set of 350 crankcases and convert them to a 500.



The inlet tract should be bigger on a 500 head (30mm vs 26mm on a 350). What carb do you have? Have you checked the engine number to see if it's actually a 500 motor?



In any case, the plug should protrude out into the combustion chamber slightly. Like you can see in the twin-plugged head in the picture below.

[stinkwheel]

Try image again.

[Natffs]

Thanks for that comprehensive reply. Yes it's a 350 as the barrel is smaller than my brothers 500. The carb is smaller too as is the header. I will clean the threads and will put a long reach plug in with a 7 heat rating

[Revband]

Short reach plugs are generally used in iron heads and long reach in aluminium heads.